A tuner performs frequency down conversion to translate a specific TV channel at a selectable RF frequency to a fixed IF frequency for further processing. Referring to FIG. 1, a conventional direct conversion tuner comprises tunable tracking band pass filter 100, local oscillator (LO) 120, mixer 110, and fixed IF band pass filter 130. Tracking filter 100 of approximately 10 to 20 MHz in bandwidth is tuned to center around the desired channel. In high side mixing operation, LO 120 is tuned to a frequency of Fch+Fif, where Fch is the center frequency of the desired channel and Fif is the IF frequency of the IF band pass filter. A low side mixing operation would use an LO frequency of Fch−Fif. Tuning control 140 produces analog or digital signals to tracking filter 100 and LO 110 according to the selected channel. Mixer 120 produces frequency terms that are the sum and difference of the LO and RF signals, and the sum and difference of harmonics of the LO and RF signals. One of these mixer term is the RF signal image at the IF frequency. IF band pass filter 130 removes the other mixer output terms. IF band pass filter 130 is typically 6 MHz wide centered at 44 MHz for a North American TV system. Other regions use different channel spacing and IF filter bandwidths.
Tracking filter 100 has a pass band response that passes one 6 MHz channel as well as some or all of several adjacent channels and may comprise other functions such as automatic gain control (AGC) and amplifiers. Tracking filter 100 provides a pre-selection function to limit the power input to mixer 120 and reduces the possible mixer term outputs. There are two possible frequencies that will produce images of the RF signal falling within the IF band pass filter after the mixing operation. The desired channel is 44 MHz below the LO frequency. An undesired channel is 44 MHz above the LO frequency. Tracking filter 100 is designed to have a deep null in the transfer function approximately 90 MHz above the center of the desired channel to provide a high degree of rejection of this undesired channel.
Tracking filters are designed for operation over the cable frequency band of 50 to 750 MHz or TV broadcast band of 50 to 800 MHz. The tracking filters have a specified rejection over the operating band but have an unspecified rejection outside the intended operating band.
The RF and post IF processing is sensitive to signals at FLO+/−FIF, and is also sensitive to frequencies at multiples of the LO frequency. Interference is caused when energy is present in the signal at (N×FLO)+/−FIF, where N is an integer. These frequencies will be translated by the mixer to the IF frequency and pass through the IF band pass filter.
Various aspects of television signals, tuners, cable propagation characteristics, and frequency usage are disclosed in W. Ciciora, et al, Modern Cable Television Technology, 1999, Morgan Kaufmann Publishers, Inc., incorporated herein by reference.
The undefined rejection of the tracking filter of tuners outside the intended signal band combined with the sensitivity of the mixer to these frequencies makes the TV receiver or set top box susceptible to energy emission present on the input signal. This creates a problem when other signals are transmitted on a cable carrying television signals.
Bit loading and power loading algorithms have been applied to peak power limited multicarrier systems, such as asynchronous digital subscriber loop (ADSL) systems. One approach is described in Enzo Baccarelli, et al., Novel Efficient Bit-Loading Algorithms for Peak-Energy-Limited ADSL-Type Multicarrier Systems, IEEE Transaction on Signal Processing, Vol. 50, No. 5, May 2002.